Noise pollution is a growing problem in society today. Everyday life presents myriad situations in which high levels of noise are the norm, for example, automobile traffic, factory workplaces, and busy shopping centers. Perhaps even more disconcerting is the fact that office environments, where relative quiet is essential to concentration, are becoming increasingly noisy.
Unfortunately, one of the major sources of increased office noise is also the source of progress and productivity: computer equipment. Personal computers, servers, and other common office computer equipment create noise and can be distracting. The single biggest sources of noise in these systems are fans. Fans are required for cooling heat-generating components such as disk drives, processors, and the power supplies. In general, the more powerful the device, the more heat energy it generates; hence, the more cooling it requires. It is an absolute must to provide sufficient cooling, otherwise the reliability of the device will suffer, and it may even break down altogether. Most fans provides a `continuous` type of noise, although some low-quality fans also provide a fluctuating noise spectrum.
Yet another source of noise is the disk drive. Disk drives provide both a `continuous` type of noise due to the rotation of the drive, and an `impulse` type of noise created by data accesses.
These two sources of noise are combined most noticeably in multiple-disk devices, such as RAID systems. RAID is an acronym first described in a University of California at Berkeley paper written by Patterson, Gibson, and Katz, entitled "A Case for Redundant Arrays of Inexpensive Disks, or RAID," published in 1987. Over time, RAID has come to refer to "Redundant Arrays of Independent Disks."
Many different disk array configurations are possible, depending on end-user requirements and the goals of the manufacturer. Each controller design has a different functionality to accomplish specific performance and data availability goals. Some disk arrays offer protection against data loss due to a drive failure. Others provide increased disk connectivity per system by allowing the disk array to appear to the host system as a single large, logical device. For systems which are constrained on the total number of logical devices, the array offers a vehicle to increase total disk storage per system. RAID systems offer large amounts of storage in relatively small cabinets, resulting in more storage per unit of floor space. With high density, small form factor disk drives all sharing the same controller and power supply, an array offers substantial floor space savings over more conventional storage alternatives. Finally, although a major function of the disk array is to provide a large amount of storage at a high level of data availability, performance gains may also be realized for certain system applications through use of an intelligent controller managing the operation of several disk drives.
With their obvious advantages, disk arrays are becoming increasingly common in many office environments. This correspondingly increases the criticality of reducing the amount of cooling fan noise associated with such devices.
It is typical that users of computer-related devices have two contradictory demands: they want their equipment to be more powerful and quieter. However, greater power usually means more heat, and more heat means more cooling is required, which usually means a more powerful (and noisier) fan is needed. This dilemma has led to the development of several approaches to reducing cooling fan noise in other types of computer devices.
Of course, the most obvious and ideal solution would be to eliminate the cooling fan altogether. However, more powerful devices generate more heat and require more cooling. Unfortunately, no devices have appeared that can remove heated air as quickly and efficiently as fans. A corollary of fan elimination involves reducing power consumption. As a long term goal, some computer manufacturers are working towards producing devices that consume less power, and are optimizing the thermal design of their devices. While significant progress is being made in these areas, most devices still require a cooling fan.
Another approach is to design and use fans that produce less noise, or to use low-noise fans and fan speed control circuits. Unfortunately, higher-capacity fans are inherently noisier, and are often necessary in more powerful devices. Fan speed control units reduce the rotational speed of the fans when the ambient temperature is low, or when the device is operating under decreased loads. However, there are limits as to how much fans can be slowed down before affecting their reliability.
Another approach employs the use of alternative cooling techniques, for example, natural convection, heat ducts, and directed air flow. Natural convection allows air to circulate through holes in the device housing. This is currently a viable solution only for low-power devices which represent only a very small portion of the market. Heat ducts, or heat pipes, soak away heat from a particular component to a radiator. Directed air flow cools heat generating components more than other, cooler components in the device. Each of these cooling techniques allows a substantial reduction in noise, but cannot yet replace the power and efficiency of cooling fans.
While at present noise reduction is largely a function of consumer preference, governments and other regulatory bodies are beginning to recognize the need to set limits on office noise. For example, European Union has issued European Norm EN31690:1992 (ISO/DIS 11690) gives guidelines for the design of low-noise workplaces. It recommends a noise exposure limit of 55 dB(A) (Lp) for normal office work, and 45 dB(A) for conference rooms and mental work. The Swiss Caisse Nationale d'Assurance recommends the same value of maximum 55 dB(A) as EN31690. The Swedish Labor Union TCO also recommended a target value of less than 45 dB(A) in office environments.
In summary, the demand for ever more powerful computer devices has spawned a corresponding need to cool the devices that meet these demands. As more the capacity for cooling increases, so does the noise produced by known cooling devices, such as fans. Increased noise levels are not only undesirable from a consumer standpoint, but are more commonly becoming subject to regulatory scrutiny. Unfortunately, known modifications and alternatives to cooling fans have proven to be inadequate to reduce noise levels sufficiently.
It is apparent from the foregoing that the need exists for some method and apparatus that will reduce the amount of noise emanating from computer devices such as disk arrays while permitting the use of sufficiently powerful cooling fans.